In recent years, numerous systems and processes have been developed in the metal refining industry for removal of insoluble sulfides from metal-containing ores. Particular advances have been made in the removal of sulfides by conversion to soluble sulfates through biochemical or biological oxidation, hereinafter referred to as a biochemical oxidation process or system. The soluble sulfates can then be readily separated from the remaining ore to allow for a subsequent efficient removal and recovery of the valuable ore metals such as gold, copper, or nickel. Sulfides occlude desired metal (e.g. Au) and prevent its recovery by conventional means (e.g., CN leach). Oxidation of sulfides "frees" metal so that the percentage of recovery can be increased.
In an industrial-scale biochemical oxidation system, a liquid slurry of an ore is prepared and fed into a suitably sized biochemical oxidation reaction vessel together with necessary biochemical oxidation nutrients, and a liquid suspension or dispersion of microorganisms such as, for example, bacterial microbes selected to provide effective oxidation of sulfides. The microorganisms must grow and develop to a satisfactory concentration level to achieve a desired high degree of removal of insoluble sulfides during a residence time of the liquids in the reaction vessel or reaction tank. The effectiveness of microorganisms to oxidize sulfides depends significantly upon the availability of dissolved oxygen in the liquid mixture in the tank. The dissolved oxygen requirements of a biochemical oxidation process are quite large due to the high stoichiometric requirement of oxygen to oxidize the insoluble metal sulfides to the corresponding soluble metal sulfates. An example of the high oxygen requirement is the biochemical oxidation of iron sulfides to ferric sulfate and sulfuric acid as follows: EQU 4FeS.sub.2 +15O.sub.2 --&lt;2Fe.sub.2 (SO.sub.4).sub.3 +2H.sub.2 SO.sub.4 (Eq. 1)
Thus, the rate at which the microorganisms in the ore slurry can be provided with a supply of dissolved oxygen determines the rate of oxidation of the sulfides to soluble sulfates. Stated differently, a reduced amount of dissolved oxygen available to the microorganisms will result in a reduced biochemical oxidation rate and, consequently, in an increased required residence time of the slurry in the biochemical oxidation system in order to effect sufficient conversion of the sulfides to the sulfates.
Numerous aeration systems and aeration methods have been devised to increase the supply of dissolved oxygen in order to manage the oxygen uptake rate in biochemical oxidation systems of a commercial scale. For example, U.S. Pat. No. 5,102,104 to Reid et al. discloses a biological conversion apparatus in which a biological conversion medium is thoroughly mixed with a biological conversion component such as, for example, air, in a plurality of mixing assemblies disposed in a cylindrical tank which has an open top end from which air is drawn into the mixing assemblies together with the biological conversion medium. The Reid et al. biochemical oxidation system proposes a total of about 60 hours of residence time of a slurry and a biological conversion medium in a tank or in tanks to achieve an approximate recovery of about 90% of a metal contained in the slurry. In U.S. Pat. No. 5,006,320 to Reid et al., there is disclosed a microbiological oxidation process for recovering mineral values. The process is a biological oxidation of sulfide in sulfide-containing ore. The process also uses aerating of the ore slurry during the biological oxidation step, in which oxygen and carbon dioxide are provided from air to a mixing assembly substantially identical to the system described in the above referenced patent to Reid et al. U.S. Pat. No. 4,987,081 to Hackl et al. discloses a chemical/biological process to oxidize multimetallic sulfide ores. The Hackl et al. process proposes to achieve as much as a 98% sulfide oxidation when the finely ground ore is leeched in agitated air-sparged tanks, with three different types of bacteria contained in different processing stages or tanks.
In the above cited references air is used to provide the oxygen to the microbial oxidation process carried out in a biochemical oxidation system. In view of the high stoichiometric requirement for oxygen in a fully effective biochemical oxidation process, the major cost of operating a biochemical oxidation system for converting insoluble sulfides to soluble sulfates is the cost associated with supplying adequate dissolved oxygen to the liquid mixture containing the microorganisms. Additionally, the ability to economically supply an adequate level of dissolved oxygen to the microorganisms frequently limits the rate of oxidation and, therefore, increases the residence time of the liquid mixture in a tank or tanks required to substantially convert the insoluble sulfides into soluble sulfates. Thus, even an effective aeration system for aerating an ore slurry containing a biochemical oxidation medium may be limited in its effectiveness by the rate at which dissolved oxygen can be provided to the system from an air supply.
Accordingly, it is desirable to provide a biochemical oxidation system for removal of insoluble sulfides from metal ores to which an enhanced rate of dissolved oxygen can be economically provided and effectively introduced into a biochemical oxidation reactor containing a liquid mixture comprises of a liquid slurry of a metal ore and of a liquid biochemical oxidation medium.